Field of the Invention
The invention relates to a method for ascertaining distortion properties of an optical system in a measurement system for microlithography.
Prior Art
Microlithography is used for producing microstructured components, such as integrated circuits or LCDs, for example. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. The image of a mask (also referred to as a reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
A characterization of the structures on the mask is performed both with regard to present deviations of the respective structure on the mask from the desired position predefined by the design (so-called positioning error or “registration error”) and with regard to the linewidth of the structures (CD=“critical dimension”).
For determining the positioning error, various methods are known in the prior art.
By way of example, a “threshold-based” image evaluation can be applied to the structures of the aerial image, as is known from US 2012/0063666 A1.
Alternatively, by using a position measurement system, a first aerial image of a segment of the mask can be recorded and compared with a simulated second aerial image, whereupon the positioning error is then equated with the distance between the structures to be measured in the measured first aerial image and the simulated second aerial image. When ascertaining the second aerial image by simulation, effects within the optical beam path of the position measurement system (e.g., on account of the numerical aperture of the position measurement system, proximity effects, etc.) are taken into account, and the inclusion of all optical parameters of the system in the generation of the simulated aerial image during the image comparison yields a result which is to the greatest possible extent independent of said optical parameters (such that the measurement result is independent of the measurement system). Such a method (which sometimes is also designated as “die-to-database” comparison where “die” refers to the measurement image and “database” refers to the simulated image) makes it possible, owing to taking account of effects in the optical beam path, to increase the accuracy in relation to a comparison of the measured aerial image with a (direct) design image of the mask.
One problem that occurs in practice, however, is that the measurement image is deformed or distorted on account of the properties of the optical system (that is to say that a coordinate grid is not exactly at right angles on the measurement image), whereas the simulated image as an ideal simulated grid does not have this property.
One known approach for taking account of the distortion consists in the latter being calibrated or “extracted computationally”, i.e. the distortion being determined metrologically by a targeted measurement with test structures in the image field. In this case, however, the further problem occurs that the distortion taken as a basis in such a calibration is dependent on the type of structure used for calibration and, consequently, is no longer exactly valid for any other possible structures.
With regard to the prior art, reference is made for example to WO 2001/012265 A1, DE 10 2007 033 815 A1 and DE 10 2006 059 431 A1, US 2010/0104128 A1, DE 10 2007 033 815 A1 and also the publication M. Längle et al.: “Pattern placement metrology using PROVE high precision optics combined with advanced correction algorithms,” Proc. SPIE 8082, 80820J (2011).